The development of a chemical process being environment friendly is the highest priority issue in the present day, which the world community agrees also in the light of green chemistry (P. T. Anastas and J. C. Wamer, Green Chemistry: Theory and Practice (Oxford University Press, Oxford, 1998). The esterification reaction is the most fundamental and important reaction of organic synthesis (Tetrahedron. 36, 2409, 1980). There are already an enormous amount of reports concerning the esterification reaction (Tetrahedron. 36, 2409, 1980), but in most of the cases, more than 1 equivalent weight of condensing agent or activating agent is used per substrate, there are problems such as a large quantity of by-product is produced after the reaction, thereby a complicated operation of separation and purification become necessary. Thus, it should actually be avoided in the light of green chemistry and atom efficiency. On the other hand, it would be an ideal process if a direct and catalytic esterification could be conducted from an equimolar amount of carboxylic acid and alcohol. But in most cases, ester can be obtained efficiently only if either carboxylic acid or alcohol is used excessively (Synthesis. 1978, 929, 1978; Chem. Lett. 1977, 55, 1977; Chem. Lett. 1981, 663, 1981; Synthesis. 1972, 628, 1972; Tetrahedron. Lett. 12, 3453, 1971; Tetrahedron. Lett. 14, 1823, 1973; Bull. Chem. Soc. Jpn. 54, 1276, 1981; Jpn. Patent Appl. 1980, No. 55-115570; Japanese Laid-Open Patent Application No. 52-75684; J. Am. Chem. Soc. 102, 7578, 1980; Tetrahedron. Lett. 28, 3713, 1987; J. Org. Chem. 56, 5307, 1991; Chem. Lett. 1981, 1671, 1981; Bull. Chem. Soc. Jpn. 62, 2353, 1989; Chem. Lett. 1984, 1085, 1984; J. Chem. Soc., Perkin Trans./1994, 3473, 1994).
Conventionally, a polyester polymerized catalyst wherein the catalyst is comprised of one or more metal compound selected from the group of scandium, yttrium, zirconium, hafnium, and vanadium, and one or more compound selected from a group comprising a compound having the structure of Ar—O— (Ar represents an aryl group) and the like (Japanese Laid-Open Patent Application No. 2000-154241) is known as polymerization catalyst. Additionally, as a method for preparing ester, wherein the catalyst activity is high, an ester is synthesized at a high yield even by using approximately an equimolar amount of acid and alkali which are the raw materials, a high reaction speed is obtained even at a low temperature, and being an excellent method in that a very small side reaction is produced, the following method for preparing ester (Japanese Laid-Open Patent Application No. 08-71429) is also known: the method uses an ester catalyst comprising a titanium metal compound selected from the group of titanium metal such as halides, nitrate salts, carboxylate salts, alcoholates and acetylacetone-type complex, as at least one of the active ingredients, and prepare ester from carboxylic acid and alcohol.
Moreover, as a method for preparing effectively carboxylic acid ester or carboxylic acid thioester from alcohol or thiol and carboxylic acid under a mild condition, the following method is known (Japanese Laid-Open Patent Application No. 05-286894): alcohol or thiol, or alternatively its ee1yl derivatives is reacted with an equivalent amount or a little smaller amount of carboxylic acid or carboxylic acid silyl ester, and when preparing carboxylic acid ester or carboxylic acid thioester, a carboxylic acid anhydride represented by a general formula (R6CO)2O (wherein R6 represents an aryl group that may have a substitute group) coexist with cationic catalyst of catalyst amount.
Recently, compounds having increasingly complicated structure and being unstable are used as drug medicine and the like, and a method of preparing ester or thioester that progresses smoothly from an equivalent amount of carboxylic acid and alcohol or thiol is anticipated in the light of composing drug medicine. An object of the present invention is to provide a method of preparing ester or thioester that can make: a catalytic esterification reaction from an equimolar amount of carboxylic acid and alcohol; or a catalytic thioesterification from carboxylic acid and thiol of which the amount is equimolar or a little smaller compared to carboxylic acid; and to provide a method that can be expected to be an industrial method that needs an enormous amount of synthesis in the light of green chemistry.